Investigations

This section contains summaries of reports on drinking water
quality incidents and investigations.

Report on drinking water quality incident at:
Sallachy Water Treatment Works in March 2007

Scottish Water Executive Summary

Sallachy water treatment works is a simple disinfection works which serves a population of approximately 60 people, producing on average 30m3 of treated water per day.

Although there is no mains power supply to the site final water chlorine monitoring and telemetry have recently been installed and are powered by batteries.

During a routine site visit to Sallachy water treatment works on the morning of 19th March 2007, it was noticed that the final water chlorine residual monitor at Sallachy WTW was reading zero. The operator checked the final chlorine residual with a hand held analyser, which also gave a residual chlorine reading of zero. The clear water tank (CWT) was immediately shock dosed to re-establish the chlorine residual. Whilst doing this the operator noticed that the flow into the tank was much lower than normal and therefore went to inspect the intake. The water level in the intake chamber was found to be low due to stones diverting the flow away from the chamber. The stones were moved and the chamber started to refill. The intake is checked on a weekly basis.

On returning to the works the operator found that although the flow into the tank had increased, the sodium hypochlorite dosing was still not running. As the operator could not establish the cause of the problem he contacted another operator who was more familiar with the works for assistance. The second operator went to site and suspected that there could be air in the raw water main which was affecting the mechanical flow meter that controls the sodium hypochlorite dosing. This was found to be the case as once the air was purged from the main the sodium hypochlorite dosing re-started.

The cause of the disinfection failure was a partial blockage of the intake causing the raw water flow to reduce to such an extent that the flow proportionally controlled sodium hypochlorite dosing stopped. As a result a low flow of untreated water would have entered the clear water tank over the weekend of the 17th and 18th March. The works had previously been visited on the morning of Friday 16th March and there were no indications of a problem.

Telemetry alarms were generated for the low final chlorine residual at Sallachy, however a human communication error occurred between the passing out and receiving of these alarms and as a consequence the standby operative for the works was not called out.

Sallachy is due to be linked into the Kyle Regional Scheme which is currently being progressed and is currently forecast to be mained out by July 2007.

DWQR Assessment on the incident

The DWQR assessment of this incident is that it was caused by a partial blockage of the intake resulting in a reduction of the raw water flow which in turn caused the disinfection dosing pump to stop.

The site is on telemetry and a low chlorine alarm was sent from the works to Scottish Water's central control room where the staff there say that they passed the alarm on to the appropriate local person. However, the local staff say they never received an alarm.

The disinfection failure therefore continued for over 2 days until the next scheduled visit by the operator. Once on site, the operator dealt with the failure efficiently and the problem was fixed within 2 hours. However, the operator should have informed Scottish Water's Public Health Team and should have taken or arranged for water samples to be taken immediately for analysis instead of waiting until he had fixed the problem. It is therefore of no surprise that the samples that were taken, passed. DWQR is aware of actions being taken by Scottish Water centrally to address issues such as these more widely within the business and trusts that Scottish Water will reap the benefits of these in time.

This was the third such disinfection failure at this works in as many months so DWQR issued a draft Enforcement Notice to Scottish Water to address the matter by accelerating its proposal to supply Sallachy by the Kyle works by 1 June 2007.

DWQR is pleased to report that he has received confirmation from Scottish Water that this work was completed before the end of May 2007.